Recently, it has been conceived that measurements conventionally conducted by multiple ranges are conducted in a single range by combining a gas mixer as shown by Patent document 1 with a gas analyzer such as an engine exhaust gas measurement system. This has merit because not only does an operation such as automatically switching the range correlation or a measurement range become unnecessary, but also the number of the calibration gases to be used can be reduced considerably.
The gas mixer is used for taking out a gas having a predetermined concentration by mixing a sample gas whose concentration is known with a dilution gas such as, for example, a nitrogen (N2) gas or air at a predetermined ratio, and is comprised of a sample gas line where a mass flow controller is arranged and a dilution gas line where a mass flow controller is arranged.
Then these mass flow controllers (MFCs) are incorporated into a gas mixer after being calibrated by the use of a reference gas such as a nitrogen (N2) gas. The mass flow controller (MFC) is used to particularly control the flow rate of the sample gas (a component gas), and converts the target flow rate of the sample gas into the flow rate of the reference gas by the use of a conversion factor (a CF value, a gas type correction coefficient) between the reference gas and the sample gas so as to set the flow rate set value of the MFC. A conventional MFC has only one CF value for each gas type and converts the flow rate of the sample gas into the flow rate of the reference gas by the use of the single CF value for all of the flow rate set values.
However, since the CF value of the sample gas differs for each concentration or each flow rate value of the sample gas, there is a problem that an error is often generated depending on the concentration and the flow rate value, if the target flow rate of the sample gas is converted into the flow rate of the reference gas by the use of one CF value alone.
Although conceivably the mass flow controller might be used in a manner that involves a separate calibration for each gas type, it would be troublesome to conduct such a separate calibration for each of the mass flow controllers for each of numerous gas types, and thus using sample gases for calibration cannot be said to be an effective measure from a cost perspective.